Conventional driving means for rail transportation systems, including driving motors and wheels, are in practice impossible to use at speeds of above 300 mph. Problems with conventional systems include running resistance, adhesion between the rails and the wheels, wind disturbances, inertial effects, road bed irregularity, and propulsion difficulties. To overcome these problems, a variety of systems have been proposed which use electromagnetic suspension and stabilization systems, instead of wheels and mechanical suspensions, to reduce dynamic friction and, consequently, enable super high speed operation while at the same time reducing energy consumption. The amount of energy used in such a system may further be reduced by the use of superconductor technology, in which electric power is delivered through wires having virtually no internal electrical resistance, thus improving the performance, cost, safety, and environmental impact, as well as the energy efficiency of the system.
Magnetic levitation (MAGLEV) technology may be broadly categorized in terms of two types of primary suspension systems: the attractive, or electromagnetic suspension (EMS) systems, and the repulsive, or electrodynamic suspension (EDS) system.
The EMS system is exemplified by the German Transrapid System, which has been under development for about 20 years and is nearing commercial application. This system uses normal (non-superconducting) electromagnets to levitate the vehicle by attraction to ferromagnetic rails mounted on a guideway.
The present development of the Transrapid System has several significant advantages and disadvantages. On the positive side are its low energy consumption, resistance to derailing, and low magnetic field strength in the passenger cabin. Disadvantages of the system include the small clearance gap of one cm (0.4 inch) required between the magnet poles and the guideway (resulting in increases in the cost of construction and maintenance), high vehicle weight owing to the use of normal magnet systems, and limited payload of freight capability.
The Japanese, on the other hand, have built and tested an EDS system (the MLU series) at speeds approaching 300 mph. Their system, unlike the German transrapid, utilizes superconducting magnet technology and therefore operates with a large clearance gap (4 to 6 inches). However, the air-core superconducting system used by the Japanese Railway experiences a magnetic quench due to dynamic effects, suffers from the disadvantage that magnetic field levels in the vehicles are very high, and requires significant additional structure supports for the load-bearing coils. The need for supports to sustain a load on the coils results in the problem of heat loss from the coils to the warm structural supports. Also, EDS systems require a relatively complex guideway, and can only levitate when a certain speed is achieved, necessitating auxiliary wheels. An example of an air-core superconducting system for MAGLEV applications is disclosed in U.S. Pat. No. 3,913,493.
Neither of the two existing MAGLEV systems has therefore proved completely satisfactory. By sacrificing the use of ferromagnetic rails to contain the magnetic flux as in conventional EMS magnetic suspensions, air-core superconductor coils such as those used in the Japanese system present significant problems because of the large and unconstrained magnetic fields generated by the coils and also because of the loads which the coils are required to bear. While the German EMS system does not present such problems, the use of conventional electromagnets limits the attractive force possible for a given current and therefore decreases the gap size, as noted above.
A contemporary development by the present assignee is directed to an improvement in this area of technology, namely an approach incorporating the best features of both the German EMS system and the Japanese EDS system without incorporating the disadvantages of either. The contemporary development is directed to a design of an EMS system which utilizes superconductive technology and yet utilizes external magnetic field levels which are minimized.
The contemporary development is more specifically directed to a MAGLEV suspension system in which the size of the air gap between the vehicle and a guideway is maximized by using superconducting coils to increase the number of ampere-turns on the electromagnet while at the same time minimizing the load on the coils by providing an air core for the coils. In this approach, the coil may be effectively isolated from the guideway, thereby facilitating cooling of the superconducting coils and minimizing heat loss.
The objectives of the contemporary development are achieved by providing the superconducting coils with iron cores to confine the magnetic flux within the iron core and a rail provided on the guideway, the iron core serving to bear the weight of the vehicle while constraining the magnetic flux to minimize stray magnetic fields. The use of iron core superconductor magnets also allows the vehicle weight to be supported by the iron core rather than the coils, reducing heat leak problems associated with transferring large loads from the superconducting coils to the relatively warm supporting structure.